Computer chips are typically connected to computer boards, such as printed circuit boards (PCB's), by soldering. For instance, conventional computer chips have been connected to such boards in the past by soldering a plurality of pins extending from the chip to the board. Presently, consumer demands are driving the current trend in the electronics industry to make products that are compact, high in density, light, and thin. These demands have created new chip interconnection methods. One such inter-connection method is known as solder bump or ball technology. Flip chip, ball grid arrays, chip scales, and multi-chip modules each use small solder bumps underneath the chips for interconnection, typically making them superior in performance to other more conventional interconnection technologies.
The standard flip chip assembly process in a surface mount technology environment, for example, includes the following steps: flux application, die placement, solder reflow, and underfill processing. Once a defect is found in the flip chip connection after underfilling, the whole circuit board usually has to be discarded, since reworking underfilled flip chips is complicated and costly. Therefore, it is desirable to have an inspection process between the solder reflow and underfill dispensing processes to reduce the manufacturing cost.
Among various solder joint defects, detection of solder joint cracks remains a challenge to available automated nondestructive inspection techniques, such as Automated Optical Inspection (AOI), Acoustic Microscopy Imaging (AMI), and Automated X-ray Inspection (AXI). Solder joints with cracks often have intermittent connections and they often can pass functional tests or in-circuit tests, but may cause problems during normal operations. Flip chip solder joints are hidden from direct view and are difficult to access with automated optical inspection equipment. Both AMI and AXI are able to generate penetrative images, but both methods have difficulties in imaging cracks, especially cracks in the vertical direction (i.e., the direction perpendicular to the imaging plane). 3D X-ray laminography techniques used in AXI are capable of identifying defects in the vertical direction. However, these techniques are generally not suitable for in-line inspection because of their slow throughput and the complicated image interpretation algorithms necessary to evaluate the data. In addition, both AMI and AXI systems usually have high operating and equipment costs.